This invention relates to a particular process and a particular arrangement for controlling an air conditioner with an evaporator protected against icing.
Processes and arrangements of this type are used particularly in motor vehicles. Icing phenomena on the evaporator are prevented by deactivating the compressor when the evaporator temperature is too low, that is, when the evaporator temperature falls below a defined switch-off temperature. The compressor will be activated again when the evaporator temperature exceeds a given switch-on temperature which is selected to be higher than the switch-off temperature. As a result, a desired switch-off hysteresis is provided which protects the compressor from too frequent switch-off and switch-on operations during protection against icing.
From German Published Patent Application DE 195 07 667 A1, it is known for a process and an arrangement of this type to define the switch-off temperature so that it falls dynamically as a function of the output of a fan assigned to the evaporator and/or of the temperature of the air taken in by the evaporator fan with a rising fan output or a rising intake temperature. In addition, it is suggested to define the switch-on hysteresis so that it is constant or decreases with a higher fan output and/or a higher intake air temperature. Such a dynamic adaptation of the switch-off temperature and, optionally, also of the switch-on temperature permits the output potential of the air conditioner to be utilized better than at a fixedly defined switch-off temperature. The reason for this is that, in the latter case, the temperature must be selected to be sufficiently high so that icing of the evaporator is reliably avoided also in critical operating points of the system, which, in many other less critical operating conditions, leads to an unnecessarily early compressor switch-off.
Another process and another arrangement of the initially mentioned type with a dynamic protection against icing is described in European Published Patent Application EP 0 557 747 A2. The suction pressure of the compressor, or its rate of increase after deactivating the compressor, which consists of disengaging an electrically controllable coupling by which the compressor is mechanically coupled with the driving engine of a motor vehicle, is used as a relevant measurable variable for dynamic protection against icing in this process and arrangement. In this case, the compressor suction pressure corresponds to the refrigerant pressure at the evaporator outlet. The compressor is deactivated when the suction pressure falls below a defined switch-off pressure, and is activated again when the suction pressure rises above a defined switch-on pressure which is higher than the switch-off pressure. Simultaneously, the time period from the switch-off point in time until the compressor is switched on again is measured. The switch-off pressure will then be updated in a variable manner as a function of this measured time period, in which case the pressure is selected to be lower as the measured time period becomes longer.
The present invention addresses the technical problem of providing a process and an arrangement of the initially mentioned type and provides a way in which, in a comparatively simple manner, icing of the evaporator is reliably avoided and the output potential of the air conditioner is utilized as well as possible.
This invention solves this problem by providing a particular process for controlling an air conditioner with an evaporator protected against icing and a refrigerant circulating system which includes at least one compressor and an evaporator. The process includes the steps of continually sensing an evaporator temperature, deactivating the compressor when the evaporator temperature falls below a defined switch-off temperature, and activating the compressor again when the evaporator temperature exceeds a defined switch-on temperature which is higher than or equal to the switch-off temperature. An evaporator temperature gradient is determined from continuously sensing the evaporator temperature during operating phases with a deactivated compressor and/or an activated compressor, and the switch-off temperature is variably defined as a function of the evaporator temperature gradient determined so that the switch-off temperature is selected to rise with a negative evaporator temperature gradient, with an amount which increases, during operating phases with an activated compressor and/or with a falling positive evaporator temperature gradient during operating phases with a deactivated compressor.
A particular arrangement for controlling an air conditioner with an evaporator protected against icing and a refrigerant circulating system which includes at least one compressor and an evaporator is also provided. The arrangement includes an evaporator temperature sensor and a control unit to which evaporator temperature information is supplied by the evaporator temperature sensor, which deactivates the compressor when the evaporator temperature falls below a defined switch-off temperature, and which activates the compressor again when the evaporator temperature exceeds a switch-on temperature which is at least equal to the switch-off temperature. The control unit determines, from the continuously supplied evaporator temperature information, an evaporator temperature gradient during operating phases with a deactivated and/or activated compressor and variably defines the switch-off temperature as a function of the evaporator temperature gradient determined. The switch-off temperature rises with a negative evaporator temperature gradient, whose amount increases, during operating phases with an activated compressor and/or with a falling positive evaporator temperature gradient during operating phases with a deactivated compressor.
In the process, the evaporator temperature is sensed continuously and, from the temperature, the evaporator temperature gradient is determined during operating phases with a deactivated and/or an activated compressor. The compressor is deactivated when the evaporator temperature falls below a switch-off temperature which is variably defined as a function of the determined evaporator temperature. In particular, when using the evaporator temperature course during operating phases with an activated compressor, during which the evaporator temperature gradient is negative, the switch-off temperature is selected in a rising manner with an evaporator temperature gradient having an amount which increases. In contrast, when using the evaporator temperature course during operating phases with a deactivated compressor, during which the evaporator temperature gradient is positive, the switch-off temperature is selected so as to rise with a falling evaporator temperature gradient. By this approach, a true evaporator temperature control is implemented with respect to dynamic evaporator icing protection since the evaporator temperature course is fed back for determining the respective optimal compressor switch-off temperature. The arrangement is suitable for implementing this process, for the purpose of which the corresponding control unit has a suitable design.
By using the evaporator temperature as a relevant measurable variable, a dynamic icing protection control can be implemented comparatively easily. In particular, sensing of the evaporator temperature gradient, while the required precision of the icing protection measurement is given, is easier than, for example, sensing the rate of change of the compressor suction pressure. Another advantage is the option of also being able to use, for setting the switch-off temperature, the evaporator temperature gradient during the active compressor operating phases; this evaporator temperature gradient may be used either alone or in conjunction with an additional analysis of this evaporator temperature gradient during deactivation phases of the compressor. By using both types of information, as required, a redundancy or an increased precision can be provided.
In a further developed process, the switch-on hysteresis, that is, the difference between the switch-on temperature and the switch-off temperature, is also defined to be variable as a function of the determined evaporator temperature gradient such that it rises when the amount of the negative evaporator temperature gradient increases or the amount of the positive evaporator temperature gradient falls, which also applies for the switch-off temperature. In the case of a slowly falling evaporator temperature during a compressor activation phase, the switch-off temperature can therefore be lowered and the switch-on hysteresis can be selected to be small without having the evaporator ice up. In contrast, when a rapidly falling evaporator temperature is present while the compressor is activated, which corresponds to a low air conditioner load, a sufficiently high switch-off temperature is set with a larger switch-on hysteresis in order to prevent icing of the evaporator.
Advantageous embodiments of the invention are described in the following with reference to the drawings.